1. Field of the Invention
The present invention relates to a mechanism for extending a lens barrel in an optical system.
2. Description of the Related Art
In a conventional a multi-stage zoom lens barrel structure proposed in Japanese Unexamined Patent Publication No. 7-27963, a zoom lens barrel has a three-stage extending mechanism, and a fixed barrel cylinder is disposed on the outermost circumference; an inner frame is moved by a gear from a collapsed position to a wide angle position through a telephoto angle position, which are photographing standby positions, thereby to perform zooming.
According to another structure disclosed in Japanese Unexamined Patent Publication No. 8-313788, a feed screw connected to a driving system is used to move the lens barrel from the collapsed position to a setup position which enables photographing, then a rotary frame that has a gear on the outer periphery thereof is turned to carry out zooming, the gear being connected to the driving system.
The structure disclosed in Japanese Unexamined Patent Publication No. 7-27963 has a shortcoming in that, since the lens frame is moved from the collapsed position to the wide angle position through the telephoto angle position by a single cam, the lead angle of the cam increases as the extending amount is increased and the load becomes heavier.
The structure disclosed in Japanese Unexamined Patent Publication No. 8-313788 is disadvantageous in that switching between the drive of the setup feed screw and the drive for zooming must be performed using an electromagnet according to the detection output indicative of the position of the lens frame given by a position detecting sensor so as to avoid a timing problem. This has resulted in a complicated switching system and a longer switching time. Furthermore, preventive measures must be provided against the leakage of light rays at the time of setup.